1. Field of the Invention
The present invention relates to a process for producing a grain-oriented silicon steel sheet having a grain orientation of {110}&lt;001&gt;which is easily magnetized in the rolling direction, more particularly to a silicon steel sheet of this type which is free from streaks of smaller, poorly oriented grains.
2. Description of Prior Art
As is well known, a grain-oriented silicon steel sheet having excellent magnetic properties in the rolling direction can be obtained by subjecting a silicon steel sheet to secondary recrystallization. In this process an important role is played by inhibitors, such as MnS and AlN which must be effectively controlled to obtain a satisfactory final product.
In the prior art processes, steel slabs are heated to high temperatures (for example 1300.degree. C. or higher) so as to fully dissolve the inhibitor elements prior to hot rolling and the inhibitor elements are controlled in the subsequent steps including hot rolling.
The temperatures to which the slabs are heated are very much higher than the temperatures used for ordinary steel grades and this very often results in excessive grain growth. Coarse grains having the &lt;110&gt; zone grain axis parallel to the rolling direction are not fully broken down and remain in the subsequent steps including the hot rolling step. As a result, secondary recrystallization in the final annealing is incomplete and the incompletely recrystallized grains remain in the final product as streaks of smaller, poorly aligned grains which will be referred to simply as streaks hereafter in this specification.
On the other hand, if the slab is heated only to a relatively low temperature (for example, 1300.degree. C. or lower), the inhibitors are not thoroughly dissolved with the result again being incomplete secondary recrystallization. In this case, streaks appear over the entire surface of the sheet.
In recent years, the old ingot-making process has been giving way to the continuous casting process. However, continuous slabs develop a columnar structure because of the unavoidable rapid cooling and solidification that characterizes the continuous casting process. In the production of grain-oriented silicon steel sheets, therefore, abnormal coarsening of the grains is more likely to occur during the high temperature slab heating step in continuous slabs than in slabs produced by the conventional process of ingot-making and breaking down. This abnormal coarsening of the grains is the principal cause of streaks in the products after the final annealing.
For prevention of streaks in products from continuously cast steel slabs, U.S. Pat. No. 3,764,406 and No. 3,841,924 disclose a method in which a continuous slab is preliminarily heated and rolled prior to the hot rolling step, so as to prevent the coarsening of grains during the high temperature slab heating in the subsequent hot rolling step.
As, however, the industrial advantage of the continuous casting method over the ingot-making method derives mainly from the elimination of the break-down step, the preliminary heating and rolling prior to the hot rolling step required in these prior-art methods greatly reduces the significance of the continuous casting method.